Familial hypertrophic (FHC) and dilated cardiomyopathy (DCM) are distinct forms of cardiac hypertrophy that progress to heart failure. FHC is characterized by a thick, hypercontractile left ventricule, whereas DCM patients have thin and hypocontractile ventricles. Both FHC and DCM can result from genetic mutations to either cardiac myosin or actin. Thus, both FHC and DCM can be classified as diseases of the sarcomere, the most basic contractile unit of muscle. In the sarcomere, myosin, a molecular motor, interacts with actin to generate the power of the heart. This Program Project (3 Projects and 3 cores) focuses on mutations to myosin and actin. Using state-of-the-art techniques, we will characterize both FHC and DCM from the mechanics of the whole heart down to the molecular mechanics of a single contractile protein, to assess how structural alterations to these proteins affect the mechanical properties of the sarcomere, the muscle fiber, and the whole heart. Project 1 will use the laser trap to assess the force and motion generation of these mutants at the single molecule level. Project 2 will biochemically and structurally characterize these proteins, while Project 3 will study the mechanical properties of skinned muscle fibers and myofibrils from trans-genic mouse hearts with mutations in either myosin or actin. Project 2 will genetically engineer FHC and DCM muta-tions into myosin and actin using in vitro expression systems. The Technologies for Experimentation, Modeling and Analysis Core (Core B) will support the projects in their data collection, analysis, modeling, and biological preparation development. The Mouse Production and Ventricular Function Core (Core C) will generate mice with FHC and DCM mutant hearts to be studied at all anatomical levels by the various projects. This Core will also characterize the in vivo and in vitro ventricular performance of the transgenic hearts over time as the disease phenotype develops. The long term goals are: 1) to utilize FHC and DCM-related point mutations as a means of identifying key structural domains within the mutant sarcomeric proteins and to determine how these domains relate to the protein's molecular function;2) to understand how point mutations in contractile proteins alter sarcomere and cardiac function so that two drastically different cardiac hypertrophies, i.e. FHC and DCM, develop.